1. Field of the Invention
This invention relates to an acquisition and tracking system and more particularly to a digital tracking system for a remote laser designated target.
2. Description of the Prior Art
Target acquisition and tracking systems operable with optical energy devices are well known to those skilled in the art. The measure of the position of a laser designated target for example creates a complex problem due to target range and the characteristics of the laser transmitter which generates a low frequency pulse train of narrow optical pulses. An electro-optical system including a servo directed line of sight and quadrant detector are conventionally used in the respective of the target return pulses. Because the field of view of the system is relatively narrow, typically in the order to 2.degree.-4.degree., a scanning technique must be utilized which is adapted to ensure that the laser spot falls within the field of view of the detector. Once the target is captured, a servo loop is adapted to respond to tracking error signals to follow the target and accordingly null the tracking error signals.
In typical prior art systems presently known, the measure of position error is proportional and requires a complex analog receiver due to the narrow pulse width of the return signals, the variation of pulse amplitude, and hence automatic gain control with range and noise fluctuations which become a significant factor because of the wide receiver bandwidths required. There inherent limitations have recently been overcome by the use of a digital or "bang-bang" tracker. These systems rather than using a proportional error, use only the algebraic sign of the error and create fixed amplitude servo control signals which force the tracker optics to oscillate in a "limit cycle" about a null error point, whereupon the sign of the error changes as the detector sweeps through the system null or boresight. The limit cycle amplitude, however, is normally quite large particularly for targets which have an apparent velocity in acceleration relative to the line of sight of the tracking system.